Communication links between computer devices are often provided by direct cable connections or by modems that transmit data over telephone lines. Direct cable connections are expensive to install and require that each piece of equipment operate with a common communication protocol. Computer equipment linked through telephone modems also requires a common communication protocol and requires the user to either maintain or pay for the use of the connecting telephone lines.
Radio modems, such as the one described in U.S. Pat. No. 4,665,519 to Kirchner et al., use radio frequency data transmission to provide communication links between computer devices. Such systems are an alternative to direct cable connections and telephone modem systems that avoid the problems associated therewith.
Prior art radio modem systems, however, have their own set of limitations. For instance, data transmitted over radio frequency waves is subject to interference by ambient background noise existing in the transmission medium. As the level of noise in the transmission medium increases, the error rate of the data received by a radio modem increases. Errors in the data received by a radio modem can be detected by transmitting redundant code and then corrected by including still more redundant code. Error detection and correction methods are well known in the art and are described generally in George C. Clark, Jr. and J. Bibb Cain, Error Correction Coding for Digital Communications, Plenum Press, 1981. For high error rates, however, it is possible that the error detection and correction will not be sufficient to recover the original data. In such a situation, the whole packet of data will have to be retransmitted. In short, error detection and correction methods increase the number of bits and the amount of time required to transmit a packet of data. Accordingly, it is advantageous to make the error rate as small as possible.
Radio modems can be both stationary and mobile. The further that radio modems are from one another, the lower the signal to noise ratio and thus the greater the error rate of the transmitted signal. Similarly, if the transmitting power of a radio modem is reduced, for instance to conserve power or to reduce interference among closely spaced stationary units, the signal to noise ratio decreases and thus the error rate increases.
Furthermore, a cellular architecture for mobile computing places stationary units in neighboring cells. If the neighboring cells use different frequencies for the transmission of data, there is no problem with interference. Often times, however, stationary units in neighboring cells are allowed to share the same frequency. In such a situation, if the neighboring cells are too close to one another or if the stationary units are transmitting at too high a power, a problem with interference and transmission errors will occur. One solution is to move the cells further apart, but this will result in a decrease in the amount of frequency sharing that can be used in a given area.